Acyloins, or alpha-hydroxyketones, are compounds which possess an optically active C atom and which play a considerable role in the synthesis of more complex compounds, for example, and in particular, (R)-(−)-phenylacetylcarbinol (PAC), which is of great economic interest for producing ephedrine. The R enantiomer, which is formed by using Saccharomyces cerevisiae to fermentatively convert pyruvate in the presence of benzaldehyde (DE-PS 548 459 dated 1932), is required for this purpose.
In this synthesis of PAC using yeast cells, a large number of byproducts are formed as a result of the activity of the majority of the enzymes present in the yeast, and the growth of the cells is inhibited by the presence of benzaldehyde.
Even when it has been isolated from the yeast, the pyruvate decarboxylase (PDC) gives rise, in this reaction, to substantial proportions of 2-hydroxypropiophenone, which is an isomer of PAC.
Thiamine diphosphate-dependent and Mg++-dependent PDC (E.C. 4.1.1.1) is widely distributed and is found in many plants, yeasts and other fungi and in some bacteria. It catalyzes the non-oxidative decarboxylation of pyruvate to acetaldehyde, and an acyloin condensation, with the formation of alpha-hydroxyketones, takes place as a side reaction.
An enzymic reaction of this nature also takes place when an aldehyde is used as the starting compound, instead of alpha-ketocarboxylic acids, and the aldehyde which is formed by the decarboxylation can also take part in the condensation reaction as a “cosubstrate”, with the formation of homoacyloins R—CHOH—CO—R′, in which R=R′.
WO 96/37620 describes the preparation of acyloins from acetaldehyde and benzaldehyde by catalysis using a recombinantly modified pyruvate decarboxylase obtained from Zymomonas mobilis. Those enzymes in which the tryptophan residue in position 392 has been replaced with a sterically smaller residue, such as alanine, glycine, phenylalanine, leucine, isoleucine, arginine, histidine, serine or threonine, are in particular described as being suitable. However, it is reported that acetaldehyde inactivates all these enzymes.